Method for induction heating



March 4, 1969 METHOD FOR INDUCTION HEATING Filed Feb. 15, 1967 Iii- JNVENTOR. 6m! 8. Yrene BY c. s. YRENE 3,431,379 J United States Patent 2 Claims ABSTRACT OF THE DISCLOSURE A method of heating articles made of nonmagnetic material first providing a thin layer of magnetic material on the workpiece and thereafter employing electric induction to heat the article.

Background of invention There are many heating operations in which it is desirable to use induction heating rather than some other form of heating. In induction heating, the induction coil acts as a transformer primary Winding while the article to be heated forms the secondary circuit. The alternating current passes through the induction-coil induces currents by transformer action in the secondary circuit, including eddy currents. Heating results primarily from the PR losses in the secondary circuit, or article, particularly from the eddy current losses. The eddy current losses are generally concentrated near the surface of the secondary circuit and are commonly referred to as the skin efiect.

An induction heating system generally includes a high frequency AC generator connected to an induction coil. The induction coil may be a circular or an open or U- shaped configuration. In either case, the coil comprises a number of turns of wire or water-cooled tubing. It is apparent that an open or U-shaped coil will provide less inductive coupling to the secondary circuit than the circular coil.

In the past, induction heating has been limited to those situations where the article to be heated was made of a metal, either a magnetic material or an electrically conductive material. In the latter case, the good electrical characteristics of the material decreased the efficiency of induction heating and required greater power input and the more efficient inductive coupling provided by a circular coil. Since there are many operations where it is undesirable or physically impossible to place a circular coil around the apparatus to be heated or a coil had to be specially designed for a particular heating operation and in many cases left in place after heating was completed, some other less desirable method of heating has been used.

It has been a particular problem in the past to solder electrical connections or braze a pressure seal around electrical leads in a pass-thru sleeve (usually of copper) through a pressure vessel, particularly where work is performed out in the field, since the length of the electrical leads prevent the use of a circular coil and the conductive material secondary circuit for the induction heater prevents or renders undesirable the use of a U-shaped coil. Where induction heating has been used, it has required relatively long heating times to reach brazing temperatures and large power inputs.

Summary of invention In order to overcome the limitations in the prior art noted above, it is an object of this invention to provide a method of induction heating which will efiiciently heat nonmagnetic material.

It is a further object of this invention to provide a method of inductive heating which will efliciently heat electrically conductive material.

Various other objects and advantages will appear from the following description of one embodiment of the invention, and the most novel features will be particularly pointed out hereinafter in connection with the appended claims.

This invention comprises a method of heating nonmagnetic material by the steps of first plating the material with a layer of magnetic material, thereafter positioning an induction coil in inductive relationship with the material and energizing the coil.

Description of the drawings The accompanying drawings illustrate the application of the present invention, as it may be employed with reference to a hollow member, wherein:

FIG. 1 is a perspective view of a pass-thru sleeve in inductive relationship with a U-shaped induction coil; and

FIG. 2 is a cross sectional view of the pass-thru sleeve of FIG. 1 along line 22.

Detailed description The applicant has discovered that a nonmagnetic article or workpiece can be efiiciently heated with an inductive coil heating system if there is first applied a thin layer of magnetic material on the surface of the workpiece. It has been found that the layer of magnetic material provides good inductive coupling and sustains high 1 R losses. Further, the magnetic material has good heat conducting properties and readily conducts the heat generated therein to the workpiece. It is preferred that the magnetic layer be applied or plated on the article, e.g., by electroplating or some other form of plating which will provide good heat conduction between the magnetic layer and the article.

This invention can be utilized to heat efiiciently any nonmagnetic article or workpiece. The article is first coated on the outer surface thereof with a layer of magnetic material. The article can be coated on all surfaces but for the purposes of this invention description, the working or heating surface is the outer surface for all practical purposes due to the skin effect. An induction coil, either circular or U-shaped, is thereafter positioned in inductive relationship with the workpiece and connected to a source of alternating current, such as a high frequency generator. When energized by the alternating current source, the induction coil induces electric currents in the article (if it is electrically conductive) and the layer of magnetic material. Due to the skin effect, the eddy currents losses are concentrated in the magnetic material layer imparting heat to the magnetic material which in turn imparts heat by conduction to the article. If the article to be heated is a nonconductor, all currents will be induced in the magnetic material layer.

The invention can be practiced on any nonmagnetic or nonconductive material such as copper, aluminum or stainless steel or even to cure or heat a plastic article using a layer of any magnetic material disposed thereon such as iron or steel, though iron is preferred since it has greater heat producing magnetic and electric losses. As noted above, the induction coil can be either circular or U-shaped depending on the configuration and accessibility of the workpiece. A circular coil is generally more eiiicient and preferred, however, the improved inductive coupling provided by the magnetic material layer enables the eflicient use of a U-shaped coil in many applications.

The drawings illustrate one application of this invention where a circular coil could not conveniently be used because of the con-figuration of the article or workpiece and associated apparatus. It is apparent that any article, either solid or hollow, can be heated by this invention and the description below is not intended to limit the invention to the disclosed article or article configuration.

In FIGS. 1 and 2, a conventional pass-thru sleeve acts as a single turn secondary winding to U-shaped induction coil 12. Coil 12 is connected to a high frequency power supply 13. Sleeve 10 comprises a generally cylindrical member 14 which has been plated in any conventional or suitable manner with a thin magnetic layer 16. Magnetic layer 16, in FIG. 2, has been shown for purposes of illustration as having an exaggerated thickness. For purposes of this invention, the magnetic layer desirably need not be thicker than about 0.03 inch. A plurality of electrical leads 18 pass through sleeve 10 into a pressure vessel (not shown). Brazing alloy can be positioned in a manner well known in the art within sleeve 10 so that the brazing alloy will melt when heated to the brazing temperature and fill the interstices 20 between leads 18 and the interior of sleeve 10 to form a pressure seal therein upon cooling.

Member 14 is usually a nonmagnetic material such as copper or stainless steel Layer 16 is a magnetic material such as a form of iron. If it is desired, magnetic layer 16 may be removed after brazing in any suitable manner such as by etching, abrasion, etc. The brazing alloy can be a lcadtin solder or any commonly available brazing alloy such as a silver brazing alloy (45% silver, 15% copper, 16% zinc, 24% cadmium) which has a brazing temperature of about 1200 F.

In order to braze leads 18 to the interior of sleeve 10 to provide a pressure seal, coil 12 is energized by power supply 13 thus inducing secondary and eddy currents in member 14 and layer 16. Due to the skin effect most of the eddy currents and consequently the PR losses are in layer 16. The resulting heat generated in layer 16 is conducted through member 14 to the brazing alloy and leads 18.

By way of example, a copper pass-thru sleeve without a magnetic material layer was heated to the brazing alloy temperature of about 1200 F. with conventional circular and U-shaped induction coils using about the same power input for each test and the results compared with the method of this invention. A circular induction coil took a total of 2 minutes and 10 seconds to melt solder and braze the leads to the copper sleeve while the U-shaped induction coil was unable to heat the sleeve and solder to a brazing temperature after more than 3 minutes in a similar copper sleeve. With the present invention, a circular induction coil took a total of only about 8 seconds to melt solder and braze the leads to a copper sleeve having a magnetic material layer, while a U-shaped induction coil took a total of about only 15 seconds to melt the solder and to braze the leads to a similar copper sleeve having a magnetic material layer.

If it is desired to heat only a selective portion or part of an article or workpiece, the surface of the article in juxtaposition with the portion or part to be heated can be plated in any conventional or suitable manner with a thin magnetic layer. When an energized induction coil is placed adjacent the magnetic layer the selected portion or part of the article will be heated, generally to the exclusion of adjacent portions or parts.

It will be understood that various changes in the details, materials and arrangements of the parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.

What is claimed is:

1. A method for inductively heating and sealing a copper sleeve having a plurality of electrical leads therethrough comprising the steps of:

(a) applying a layer of magnetic material on the surface of said sleeve,

(b) applying fusible metal to the interior of said sleeve,

(c) positioning inductor coil heating means adjacent said sleeve, and

(d) energizing said inductor coil.

2. The method of claim 1 in which said sleeve is annular and said inductor coil is U-shaped.

References Cited UNITED STATES PATENTS 2,267,001 12/1941 Toulmin 2l910.41 2,653,210 9/1953 Becker et a1 2l99.5 2,743,345 4/1956 Seulen et a1 219l0.79 X 2,899,525 8/1959 Lederman et al. 2l9l0.41 3,118,365 l/l964 Rollo et a1 2l910.53 X 3,204,074 8/1965 Hunting 2l9l0.79 3,359,398 12/1967 Reinke et al 2l9l0.79 X

RICHARD M. WOOD, Primary Examiner.

L. H. BENDER, Assistant Examiner.

US. Cl. X.R. 

